A buck-boost voltage converter receives an unregulated input voltage and generates an increased or decreased regulated output voltage, where the target output voltage is set by component values in a feedback circuit. The buck-boost converters related to the present inventor are pulse-width modulation (PWM) converters, where the switching duty cycle of either buck or boost mode transistor switches controls the output voltage. The switching causes current through a smoothing inductor to ramp up and down as the inductor is charging and discharging.
A voltage mode converter regulates the output voltage by applying a fraction of the output voltage to a first input of an error amplifier, where the second input of the error amplifier is coupled to a fixed reference voltage. The output of the error amplifier (the error voltage) is then compared to a sawtooth waveform to turn the converter switches on and off at a duty cycle required to keep the two inputs into the error amplifier matched. Although the sawtooth frequency is typically greater than 1 MHz, the voltage feedback loop is a relatively slow loop since the output voltage is highly filtered and is slow to change.
A peak current mode control converter compares the error voltage to a varying signal directly corresponding to the instantaneous current through the inductor. The switching transistors are reset when the ramping inductor current crosses the error voltage. This is called peak inductor current control. The current feedback loop is a fast loop since the charging of the inductor is immediately stopped upon the ramp reaching a threshold. This control method provides a very fast response to transient conditions, such as short circuits and overload conditions.
A much less common control method is called average current control. In such a control method, the varying signal (directly corresponding to the instantaneous inductor current) and the error voltage are applied to inputs of a differential transconductance amplifier, and the output of the differential transconductance amplifier is filtered to substantially average the output of the amplifier (related to the average of the inductor current). The filtered signal is referred to herein as the “average inductor current demand signal,” which is not actually the average inductor current but only related to it. Such a filter may be a type II compensation network or other suitable filter. The filtered waveform is then compared to a sawtooth signal to control the switching of the converter's transistors. Such a control mode provides increased current loop gain at low frequencies and improves immunity to noise in the inductor current. Such characteristics are beneficial in certain applications.
In a buck-boost converter using average current control, the average inductor current demand signal is compared to a buck sawtooth waveform (having a peak corresponding to 100% duty cycle of buck switching transistors) and compared to a higher voltage boost sawtooth waveform (having a base voltage starting at the buck sawtooth waveform peak and a peak corresponding to 100% duty cycle of the boost switching transistors). The level of the average inductor current demand signal automatically controls whether the converter operates in the buck or boost mode and controls the duty cycle of the switching transistors.
There is some residual ripple of the average inductor current demand signal after it has been filtered by the compensation circuit. This ripple has three main effects. The current loop gain depends not just upon the sawtooth generator slopes but also on the average inductor current ripple slope. Sub-harmonic oscillation can result if the off-time ripple slope is too large. And, the buck-boost converter can jump between the two modes in the middle of a switching cycle if this residual ripple is too high. The compensation circuit cannot have too slow of a time constant or else the regulator will not adequately react to transient conditions, so traditionally some ripple must be tolerated.
What is needed is a buck-boost converter that uses average current control and does not suffer from the drawbacks mentioned above.